As shown in FIGS. 1 and 2, it is known to provide an open cavity 5 or “bathtub” at the free end 3 of a hollow blade 2, the cavity being defined by an end wall 7 that extends over the entire end of the blade, and by side walls in the form of two rims 9 and 10 that extend between the leading edge 12 and the trailing edge 14 of the blade, one of the two rims 9 extending the pressure side wall 8 of the blade while the other rim extends the suction side wall 11 of the blade. These rims are referred to herein as the pressure rim and the suction rim.
The rubbing surfaces between the free end 3 of each blade and the annular surface of the turbine casing 16 that surrounds the blades, as shown in FIG. 2, are thus restricted to the rims 9 and 10 so as to protect the body of the blade, and more particularly its end wall 7. Furthermore, the rims 9 and 10 serve to optimize the clearance J between the free end of the blade 3 and the casing 16, and thus limit the amount of gas that can pass from the pressure side to the suction side, thereby generating aerodynamic losses that are harmful to the efficiency of the machine.
Because of the high temperatures of the gas passing through the turbine and the high speeds of rotation of the blades, it is necessary to cool the end rims 9 and 10 so as prevent them from deteriorating under the combined effect of friction and heat. Thus, cooling channels are generally provided to connect the internal cooling passage 18 of the hollow blade to the open cavity 5 so as to deliver cool air to the rims 9 and 10.
Furthermore, recent studies have shown that the absence of the pressure rim 9 makes it possible to overcome problems of this rim deteriorating while nevertheless still guaranteeing turbine efficiency that is as good as or even better than the efficiency of a blade having two rims, one on the pressure side and another on the suction side.
However, the absence of a pressure rim 9 makes it necessary to improve the cooling of the suction rim 10, since it is no longer protected from hot gas by the pressure rim 9.
In conventional blades having two rims as described in documents EP 0 816 636 B1 and EP 1 270 873 A2, the channels dedicated to cooling the suction rim cool it poorly, either because they are located too far away from said rim, or because they open out in the vicinity of the top end of the rim. At best, they cool only the top end of the suction rim.
Document EP 1 422 382 A2 presents a solution for improving the cooling of the suction rim of a blade that does not have a pressure rim. That solution consists in making notches in the side wall of the suction rim facing the open cavity and impacted by the hot gases. Said notches extend substantially from the base of the suction rim to the top end of said rim, and cooling channels are pierced at the bottoms of these notches down to the internal cooling passage in the blade. The main drawback of those notches are that they weaken the suction rim, in particular at its top end, and they confine the cool air so that the wall portions that are situated between the notches are cooled not at all (or very poorly). In addition, depending on the shapes of the notches, it can be difficult to drill the cooling channels, and that operation can require specific equipment.